(1) Field of the Invention
This invention relates to a method and apparatus for optically sensing the presence of an object, like a sheet of paper, at a working location, like a print station, in a machine, like a printer, and it also relates to a method and apparatus for determining the skewness, if any, of the object in being fed to the working location.
(2) Background Information
There are several problems associated with feeding an object to a working location. For example, in the printer art, some of these problems are as follows:
(a) The sensing and aligning of various widths of forms. Because of the various widths of forms, it is difficult to use standard interrupter modules. The sensors used in the modules need to be mounted in such a manner as to detect the smallest forms without interfering with the detection of the largest forms.
(b) The sensing and aligning of documents having varying thicknesses. This is one of the most difficult problems to overcome with fixed gain optical systems in printers. The manufacturing variations such as alignment, parameter variations, dust, scratches, and degradation of the optical parts in the optical systems can cause degradation errors. This problem is compounded when the optical parts or light source detector pairs are mounted on separate moveable parts of the printer.
(c) The lifetime degradation of components and system. Another difficult problem is that of designing a system critical enough to sense all thicknesses of objects from very thin to very thick paper, for example, and to maintain the stability of the system through mechanical alignment change, component degradations, dust collection, and other life factors.
(d) Special design problems. In some situations, it is necessary that the item being moved be moved beyond the point at which its leading edge is detected. This problem precludes the use of standard interrupter devices which would be mounted in the path of the item being moved and thereby prevent movement of the item beyond the interrupter devices.